In vehicle-mounted loading cranes, the hydraulic cylinder which serves to lift the crane arm is secured by one end to the crane post while the opposite end thereof is secured to the crane arm or boom at a distance from the point of attachment of the arm to the crane post. The point of attachment of the lifting cylinder to the crane arm in this case consists of the end of a pivotal lever the operative length of which is at its maximum when it is directed at 90.degree. the lifting cylinder but which becomes smaller as this angle gets smaller or larger, i.e. when the crane arm pivots to its highest or lowest positions.
The product of the operative lever arm length and the power of the lifting cylinder gives the crane arm the required lifting moment. It is evident that the lifting moment is reduced when the lever arm is short and may cease completely, when the lever length approaches the value zero.
The crane arm should be capable of operating within an elevation range of appr. 160.degree. and consequently its lifting capacity is limited at the upper and the lower parts of this range. However, it is necessary that the crane arm possesses sufficient lifting capacity also when it assumes its maximum and minimum angular elevation positions, both in order to be capable of lifting the load within the working area of the crane and also in order to be capable of lifting the crane arm to and away from a low parked position.
To solve this problem, various methods have been suggested. The one closest at hand is to use an excessively strong lifting cylinder, whereby the crane arm achieves sufficient lifting capacity in the extreme positions of its elevation area. However, the disadvantage of this arrangement is that the crane will be excessively strong also in more normal arm positions, with the possible consequence of overloading.
Another solution is disclosed in the Austrian Pat. No. 309,735 according to which a valve which is controlled by the pivotal movement of the air cylinder, serves to vary the pressure to the air cylinder, increasing it to the required extent to compensate for the reduced lever ratio.
The lifting cylinder could also be connected to a double link member as disclosed in the Swedish Pat. No. 308,786. Owing to this arrangement, the lifting arm is given a rather uniform lifting moment over the major part of the elevation area. However, double links of this kind, when used with large vehicle-mounted loading cranes, are so bulky that the loading tool cannot be parked on the vehicle proper without exceeding the maximum allowable vehicle width. For this reason, it has become necessary, when using cranes of this kind to arrange the crane post tiltable in relation to the crane base, allowing it to be folded to the position required to permit parking of the gripping tool within the allowable vehicle width. This arrangement is expensive and complicated.